Heat-shrinkable layered film and package made with the same

ABSTRACT

An object of the invention is to provide a heat-shrinkable multi-layer film including a layer formed of a poly(carboxylic acid) polymer (A), a layer formed of a polyvalent metal compound (B), and a heat-shrinkable support film. The invention provides a heat-shrinkable multi-layer film including a heat-shrinkable support film (base film) and, on at least one surface of the base film, at least one layer structure including a layer (a) formed of a poly(carboxylic acid) polymer (A) and a layer (b) formed of a polyvalent metal compound (B), the layers (a) and (b) being in contact with each other, and the multi-layer film exhibiting a percent thermal shrinkage of 3 to 90%. The invention also provides a packaged product obtained by packaging an object with the heat-shrinkable multi-layer film, and a heat-shrinkable label.

TECHNICAL FIELD

The present invention relates to a heat-shrinkable multi-layer film, andto a packaged product obtained by packaging an object with the film andsubjecting the thus-packaged object to thermal shrinkage treatment.

BACKGROUND ART

In a conventional hermetic container requiring oxygen-barrier property,a container member and a cover member are formed of a materialexhibiting oxygen-barrier property. In some cases, such a hermeticcontainer per se is protected with a stretchable packaging film or astretchable/shrinkable packaging film. In the case of packaging of anobject without using a container member and a cover member, the objectis packaged directly with a film exhibiting oxygen-barrier property andstretchability/shrinkability. Japanese Patent Application Laid-Open(kokai) No. 2001-341201 discloses, as a film exhibiting oxygen-barrierproperty and shrinkability, a barrier shrink film which is formed of analiphatic polyamide and a xylylene-containing polyamide and exhibitsspecific percent shrinkage and oxygen permeability under specificconditions.

Such a film exhibiting shrinkability and gas-barrier property hasconventionally been employed for packaging, and packaging filmsexhibiting various characteristics have been proposed.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a heat-shrinkablemulti-layer film comprising a layer formed of a poly(carboxylic acid)polymer; a layer formed of a polyvalent metal compound; and aheat-shrinkable support film.

The present inventors have conducted studies on a gas-barriermulti-layer film including a layer formed of a poly(carboxylic acid)polymer and a polyvalent metal compound (Japanese Patent Application No.2002-121246), and have found that when the gas-barrier multi-layer filmis formed on a heat-shrinkable base film, the thus-formed multi-layerfilm exhibits heat shrinkability with impairing gas-barrier property.The present invention has been accomplished on the basis of thisfinding.

Accordingly, the present invention provides a heat-shrinkablemulti-layer film comprising a heat-shrinkable support film (base film)and, on at least one surface of the base film, at least one layerstructure including a layer (a) formed of a poly(carboxylic acid)polymer (A) and a layer (b) formed of a polyvalent metal compound (B),the layers (a) and (b) being in contact with each other, and themulti-layer film exhibiting a percent thermal shrinkage of 3 to 90%. Thepresent invention also provides a heat-shrinkable multi-layer filmcomprising a heat-shrinkable support film (base film) and, on at leastone surface of the base film, at least one layer structure including alayer (a) formed of a poly(carboxylic acid) polymer (A), and apolyvalent-metal-compound-containing resin layer formed of a polyvalentmetal compound (B) and a resin, the layer (a) and the resin layer beingin contact with each other, wherein the multi-layer film exhibits apercent thermal shrinkage of 90% or less, and the base film exhibits apercent thermal shrinkage of 3 to 90%. The present invention alsoprovides a packaged product obtained by packaging an object with theaforementioned heat-shrinkable multi-layer film; and a heat-shrinkablelabel comprising the aforementioned heat-shrinkable multi-layer film.

BEST MODE FOR CARRYING OUT THE INVENTION

A heat-shrinkable multi-layer film according to the present invention(hereinafter may be abbreviated as “the multi-layer film”) includes aheat-shrinkable support film (base film) and, on at least one surface ofthe base film, at least one layer structure including a layer (a) formedof a poly(carboxylic acid) polymer (A) and a layer (b) formed of apolyvalent metal compound (B), the layers (a) and (b) being in contactwith each other. Generally, shrinkage of the entirety of theheat-shrinkable multi-layer film is determined by the shrinkage of theheat-shrinkable support film (base film).

The poly(carboxylic acid) polymer (A) employed in the present inventionmay be an existing poly(carboxylic acid) polymer. The term “existingpoly(carboxylic acid) polymer” refers a polymer containing in themolecule thereof two or more carboxyl groups. Specific examples of theexisting poly(carboxylic acid) polymer include homopolymers orcopolymers formed from an α,β-monoethylenic unsaturated carboxylic acid;copolymers formed from an α,β-monoethylenic unsaturated carboxylic acidand an ethylenic unsaturated monomer; and acidic polysaccharidescontaining in the molecule a carboxyl group, such as alginic acid andpectin. These poly(carboxylic acid) polymers (A) may be employed singlyor in combination of two or more species. Typical examples of theα,β-monoethylenic unsaturated carboxylic acid include acrylic acid,methacrylic acid, itaconic acid, maleic acid, fumaric acid, and crotonicacid.

Typical examples of the ethylenic unsaturated monomer capable of beingcopolymerized with such an α,β-monoethylenic unsaturated carboxylic acidinclude olefins such as ethylene and propylene; saturated carboxylicacid vinyl esters such as vinyl acetate; alkyl acrylates; alkylmethacrylates; alkyl itaconates; acrylonitrile; halogen-containingmonomers such as vinyl chloride, vinylidene chloride, vinyl fluoride,and vinylidene fluoride; and aromatic vinyl monomers such as styrene. Inthe case where the poly(carboxylic acid) polymer (A) is a copolymerformed from an α,β-monoethylenic unsaturated carboxylic acid and asaturated carboxylic acid vinyl ester (e.g., vinyl acetate), thecopolymer may be subjected to saponification before use, to therebyconvert saturated carboxylic acid vinyl ester moieties into vinylalcohol.

In the case where the poly(carboxylic acid) polymer (A) is a copolymerformed from an α,β-monoethylenic unsaturated carboxylic acid and anethylenic unsaturated monomer, from the viewpoints of gas-barrierproperty, high-temperature water vapor resistance, and hot waterresistance of the multi-layer film of the present invention, thecompositional proportion of the α,β-monoethylenic unsaturated carboxylicacid in the copolymer is preferably 60 mol % or more, more preferably 80mol % or more, much more preferably 90 mol % or more, most preferably100 mol % (i.e., the poly(carboxylic acid) polymer (A) is a polymerformed solely from the α,β-monoethylenic unsaturated carboxylic acid).

In the case where the poly(carboxylic acid) polymer (A) is a polymerformed solely from the α,β-monoethylenic unsaturated carboxylic acid,examples of the polymer include homopolymers or copolymers formedthrough polymerization of at least one polymerizable monomer selectedfrom the group consisting of α,β-monoethylenic unsaturated carboxylicacids described above as typical examples, and mixtures of suchhomopolymers or copolymers. Preferably, there are employed homopolymersor copolymers formed through polymerization of at least onepolymerizable monomer selected from among acrylic acid, maleic acid, andmethacrylic acid, and/or mixtures of such homopolymers or copolymers.More preferably, there are employed polyacrylic acid, polymethacrylicacid, polymaleic acid, and mixtures thereof. In the case where thepoly(carboxylic acid) polymer (A) is a substance other than a polymerformed from an α,β-monoethylenic unsaturated carboxylic acid monomer;for example, an acidic polysaccharide, alginic acid is preferablyemployed.

So long as properties such as a gas-barrier property and heatshrinkability of the multi-layer film are not impaired, the entirety ora portion of carboxyl groups contained in the molecule of thepoly(carboxylic acid) polymer (A) may be formed into a metal salt by useof a compound of a monovalent metal (e.g., sodium or potassium), and theresultant poly(carboxylic acid) polymer may be employed singly or incombination with another poly(carboxylic acid) polymer. Alternatively,the poly(carboxylic acid) polymer (A) may be employed in combinationwith the aforementioned monovalent metal compound.

No particular limitation is imposed on the number average molecularweight of the poly(carboxylic acid) polymer (A), but, from the viewpointof film formability, the number average molecular weight is preferably2,000 to 1,000,000, more preferably 10,000 to 500,000, most preferably30,000 to 300,000. When the number average molecular weight isexcessively small, a coating film is difficult to form, whereas when thenumber average molecular weight is excessively large, coating isdifficult to perform.

The polyvalent metal compound (B) employed in the present inventionencompasses a polyvalent metal element having a valence of 2 or more,and a compound of such a polyvalent metal element. Specific examples ofthe polyvalent metal include alkaline earth metals such as beryllium,magnesium, and calcium; transition metals such as titanium, zirconium,chromium, manganese, iron, cobalt, nickel, copper, and zinc; andaluminum. Specific examples of the polyvalent metal compound includeoxides, hydroxides, carbonates, organic acid salts, and inorganic acidsalts of the aforementioned polyvalent metals; ammonium complexes of thepolyvalent metals; secondary to quaternary amine complexes of thepolyvalent metals; and carbonates and organic acid salts of suchcomplexes. Examples of the organic acid salts include acetates,oxalates, citrates, lactates, phosphates, phosphites, hypophosphites,stearates, and monoethylenic unsaturated carboxylic acid salts. Examplesof the inorganic acid salts include chlorides, sulfates, and nitrates.Other examples of the polyvalent metal compound include alkyl alkoxidesof the aforementioned polyvalent metals.

These polyvalent metal compounds may be employed singly or incombination of two or more species. Of these polyvalent metal compounds,the polyvalent metal compound (B) employed in the present invention ispreferably a divalent metal compound, from the viewpoints of gas-barrierproperty, high-temperature water vapor resistance, hot water resistance,and productivity of the multi-layer film of the present invention. Morepreferably, the polyvalent metal compound (B) is an oxide, hydroxide, orcarbonate of an alkaline earth metal, cobalt, nickel, copper, or zinc;an ammonium complex of cobalt, nickel, copper, or zinc; or a carbonateof such a complex. Much more preferably, the polyvalent metal compound(B) is an oxide, hydroxide, or carbonate of magnesium, calcium, copper,or zinc; an ammonium complex of copper or zinc; or a carbonate of such acomplex.

In the case where the polyvalent metal compound (B) to be employed is inthe form of particles, preferably, the particle size is small, from theviewpoint of transparency of the multi-layer film. Furthermore, from theviewpoint that the below-described coating mixture for producing themulti-layer film of the present invention is more uniformly prepared,preferably, the polyvalent metal compound is in the form of particleshaving a small particle size. The average particle size of thepolyvalent metal compound is preferably 5 μm or less, more preferably 1μm or less, particularly preferably 0.1 μm or less, most preferably 0.05μm or less.

When the average particle size of the polyvalent metal compound isexcessively large, the resultant multi-layer film may encounterdifficulty in exhibiting gas-barrier property. The polyvalent metalcompound (B) is preferably employed in the form of a mixture with thebelow-described specific resin, from the viewpoints of coatingperformance of the compound, as well as adhesion between the compoundand the surface of an object to be coated. Particularly in the casewhere the layer (b) formed of the polyvalent metal compound (B) is apolyvalent-metal-compound-containing resin layer, when the multi-layerfilm is subjected to thermal shrinkage treatment, gas-barrier propertyof the film is not deteriorated but rather improved, which is preferred.Preferred examples of the resin constituting thepolyvalent-metal-compound-containing resin include resins employed forcoating materials, such as alkyd resin, melamine resin, acrylic resin,nitrocellulose, urethane resin, polyester resin, polyether resin,phenolic resin, amino resin, fluorocarbon resin, and epoxy resin. Ofthese, polyester resin and polyether resin are preferred, from theviewpoints of coating performance, adaptability to shrinkage of the basefilm, and flexibility.

No particular limitation is imposed on the type of the support filmemployed in the present invention, so long as the film is a plastic filmhaving the below-described properties. Specific examples of the materialof such a plastic film include chlorine-containing polymers such aspolyvinyl chloride and polyvinylidene chloride, and copolymers ofmonomers constituting such polymers; polystyrene polymers; polyesterpolymers such as polyethylene terephthalate and polyethylenenaphthalate, and copolymers thereof; polyamide polymers and copolymersthereof, such as nylon 6, nylon 66, nylon 12, nylon 6/66 copolymers, andnylon 6/12 copolymers; vinyl-acetate-containing copolymers such asethylene-vinyl acetate copolymers and ethylene-vinyl alcohol copolymers;and olefin polymers such as low-density polyethylene, linear low-densitypolyethylene, and polypropylene, and copolymers thereof. The supportfilm may be a heat-shrinkable stretched sheet or film formed of such aplastic material.

The heat-shrinkable support film (base film) determines the heatshrinkability of the entirety of the heat-shrinkable multi-layer film ofthe present invention. The percent thermal shrinkage of the support filmis more preferably 3 to 90%, more preferably 5 to 90%, most preferably 5to 70%, as measured in at least one direction. The present inventorshave found that when the percent thermal shrinkage of the support filmexceeds 90%, difficulty is encountered in producing the gas-barriermulti-layer film of the present invention while attaining a uniform filmthickness, and difficulty is also encountered in maintaining andimproving gas-barrier property of the multi-layer film after thermalshrinkage treatment. In order to bring the multi-layer film into closecontact with an object to be packaged after thermal shrinkage treatment,and to maintain gas-barrier property of the film after packaging of theobject, the percent thermal shrinkage of the support film is preferably3% or more.

Gas-barrier property of the entirety of the multi-layer film is securedby means of the gas-barrier multi-layer structure including the layer(a) formed of a poly(carboxylic acid) polymer (A) and the layer (b)formed of a polyvalent metal compound (B). Percent thermal shrinkage ofthe multi-layer film of the present invention is 3 to 90%, preferably 5to 90%, more preferably 5 to 80%, most preferably 5 to 70%, as measuredin at least one direction. When the percent thermal shrinkage asmeasured in at least one direction is 0 to 10%, the percent thermalshrinkage as measured in a direction perpendicular to said one directionis preferably 20% or more, more preferably 30% or more, most preferably40% or more. The upper limit of the percent thermal shrinkage asmeasured in the perpendicular direction is about 90% or less.

Thermal shrinkage treatment is preferably carried out in, for example,hot water, vapor, steam, or hot air. As used herein, the term “percentthermal shrinkage” refers to, unless otherwise specified, the percentthermal shrinkage of the multi-layer film or the support film measuredafter the film is immersed in hot water at 90° C. for 30 seconds.

Next will be described the method for producing the multi-layer film ofthe present invention. A coating method is employed for forming, on theheat-shrinkable support film, the layer (a) containing a poly(carboxylicacid) polymer (A) and the layer (b) containing a polyvalent metalcompound (B).

As used herein, the term “coating method” is a method in which a coatingliquid containing a poly(carboxylic acid) polymer (A) and a solvent, ora coating liquid containing a polyvalent metal compound (B) and asolvent is applied onto the support film, and then the solvent isremoved through evaporation or a similar technique, to thereby form thelayer (a) or the layer (b) on the support film. The coating method isspecifically performed by use of, for example, a coater or a printingapparatus. Examples of the coater, printing apparatus, and coatingtechnique which may be employed include coating techniques such as adirect gravure technique, a reverse gravure technique, a kiss reversegravure technique, and an offset gravure technique; and coaters employedin these techniques, such as a gravure coater, a reverse roll coater, amicro-gravure coater, an air knife coater, a dip coater, a bar coater, acomma coater, and a die coater. The coating method encompasses a methodin which a coating liquid containing a monomer of the poly(carboxylicacid) polymer (A) is applied onto the support film, and the monomer ispolymerized through irradiation with UV rays or electron beams, tothereby form the layer (a); and a method in which a monomer of thepolymer (A) is deposited onto the support film while the monomer ispolymerized through irradiation with, for example, electron beams, tothereby form the layer (a). Also, the coating method encompasses amethod in which the polyvalent metal compound (B) is deposited onto thesupport film through vapor deposition, sputtering, ion plating, or asimilar technique, to thereby form the layer (b) containing the compound(B).

After a coating liquid containing a poly(carboxylic acid) polymer (A)and a solvent, or a coating liquid containing a polyvalent metalcompound (B) and a solvent is applied onto the support film, the solventis evaporated for drying. No particular limitation is imposed on themethod for evaporating the solvent. Examples of the evaporation methodwhich may be employed include a method in which the solvent is naturallyevaporated, a method in which the solvent is evaporated in an oven whosetemperature is regulated at a predetermined level, and a method in whichthe solvent is evaporated in a drying apparatus equipped with theaforementioned coater, such as an arch dryer, a floating dryer, a drumdryer, or an infrared dryer. The evaporation conditions may bearbitrarily determined, so long as the support film, the layer (a)formed of a poly(carboxylic acid) polymer (A), or the layer (b) formedof a polyvalent metal compound (B) is not damaged by heat.

No particular limitation is imposed on the order for applying, onto thesupport film, a coating liquid containing a poly(carboxylic acid)polymer (A) and a solvent, and a coating liquid containing a polyvalentmetal compound (B) and a solvent, so long as at least one layerstructure including the layer (a) and the layer (b) which are in contactwith each other is formed. Each of the layers (a) and (b) may beprovided in a plurality of numbers, and in this case, layers (a) andlayers (b) may be laminated alternately, or may form a sandwichstructure. No particular limitation is imposed on the total of thethicknesses of the layer(s) (a) and the layer(s) (b) formed on thesupport film, but the total thickness is preferably 0.002 μm to 1 mm,more preferably 0.02 μm to 100 μm, much more preferably 0.1 μm to 20 μm.

The thickness of a single layer (a) is preferably 0.001 μm to 200 μm,more preferably 0.01 μm to 50 μm, much more preferably 0.05 μm to 10 μm.The thickness of a single layer (b) is preferably 0.001 μm to 800 μm,more preferably 0.01 μm to 50 μm, much more preferably 0.05 μm to 10 μm.

The ratio of the total thickness of a single layer (a) and a singlelayer (b) which are in contact with each other (hereinafter acombination of the layers (a) and (b) may be called a “gas-barrierlayer”) to the thickness of the support film; i.e., (gas-barrier layerthickness/support film thickness), is preferably 0.001 to 0.5, morepreferably 0.002 to 0.3, most preferably 0.004 to 0.2. When thethickness ratio is less than 0.001, gas-barrier property of theresultant multi-layer film may be deteriorated, whereas when thethickness ratio exceeds 0.5, cracking may occur in the gas-barrier layeror wrinkles may be formed on the multi-layer film during thermalshrinkage of the film, leading to deterioration of the transparency ofthe film.

The coating liquid containing a poly(carboxylic acid) polymer (A) and asolvent can be prepared by dissolving or dispersing a polymer (A) in thesolvent. No particular limitation is imposed on the solvent to beemployed, so long as it can uniformly dissolve or disperse thepoly(carboxylic acid) polymer (A). Specific examples of the solventinclude water, acetone, methyl alcohol, ethyl alcohol, isopropylalcohol, dimethyl sulfoxide, dimethylformamide, and dimethylacetamide.The poly(carboxylic acid) polymer (A) may be readily reacted with thepolyvalent metal compound (B) in an aqueous solution, resulting information of non-uniform precipitates. Therefore, in the case where thecoating liquid containing a poly(carboxylic acid) polymer (A) and asolvent is applied onto the layer (b) formed of a polyvalent metalcompound (B), when the solvent is water, the poly(carboxylic acid)polymer (A) may be reacted with the polyvalent metal compound duringapplication of the coating liquid, resulting in formation of non-uniformprecipitates. Therefore, the solvent employed in the coating liquid ispreferably a non-aqueous solvent, or a mixture of a non-aqueous solventand water.

The coating liquid may optionally contain, in addition to thepoly(carboxylic acid) polymer (A) and the solvent, an additive such as apolymer other than the polymer (A) (e.g., polyvinyl alcohol), asoftening agent (e.g., glycerin), a stabilizer, an anti-blocking agent,a tackifier, or an inorganic layered compound (e.g., montmorillonite),so long as such an additive does not impede gas-barrier property of themulti-layer film of the present invention, which is a final product. Thetotal amount of such additives to be added is preferably 5 wt. % orless, more preferably 3 wt. % or less, most preferably 1 wt. % or less,on the basis of the amount of the poly(carboxylic acid) polymer (A)contained in the coating liquid.

Similar to the case described above, the coating liquid may contain amonovalent metal compound, so long as the compound does not impedegas-barrier property of the multi-layer film of the present invention,which is a final product. The amount of the poly(carboxylic acid)polymer (A) contained in the coating liquid is preferably 0.1 to 50 wt.%, more preferably 0.5 to 30 wt. %, most preferably 1 to 10 wt. %. Whenthe poly(carboxylic acid) polymer (A) content is low, a coating film isdifficult to form, whereas when the polymer content is excessively high,coating is difficult to perform.

The coating liquid containing the polyvalent metal compound (B) and asolvent can be prepared by dissolving or dispersing the compound (B) inthe solvent. No particular limitation is imposed on the solvent to beemployed, so long as it can uniformly dissolve or disperse thepolyvalent metal compound (B). Specific examples of the solvent whichmay be employed include water, methyl alcohol, ethyl alcohol, isopropylalcohol, n-propyl alcohol, n-butyl alcohol, n-pentyl alcohol, dimethylsulfoxide, dimethylformamide, dimethylacetamide, toluene, hexane,heptane, cyclohexane, acetone, methyl ethyl ketone, diethyl ether,dioxane, tetrahydrofuran, ethyl acetate, and butyl acetate. As describedabove, the poly(carboxylic acid) polymer (A) may be readily reacted withthe polyvalent metal compound in an aqueous solution, resulting information of non-uniform precipitates. Therefore, in the case where thecoating liquid containing the polyvalent metal compound (B) and asolvent is applied onto the layer (a) formed of a poly(carboxylic acid)polymer (A), when the solvent is water, the poly(carboxylic acid)polymer (A) may be reacted with the polyvalent metal compound duringapplication of the coating liquid, resulting in formation of non-uniformprecipitates. Therefore, the solvent employed in the coating liquid ispreferably a non-aqueous solvent, or a mixture of a non-aqueous solventand water.

The coating liquid containing the polyvalent metal compound (B) and asolvent may optionally contain, in addition the compound (B) and thesolvent, an additive such as a resin, a dispersant, a surfactant, asoftening agent, a stabilizer, a film-forming agent, an anti-blockingagent, or a tackifier. Particularly, in order to improve thedispersibility and coating performance of the polyvalent metal compound,preferably, a resin which is soluble in the solvent employed is added tothe coating liquid.

In the coating liquid, the ratio by weight of the polyvalent metalcompound (B) to the resin (R); i.e., (B)/(R), is preferably 0.1 to 9,more preferably 0.1 to 5, most preferably 0.2 to 5. When the ratio(B)/(R) is high, adhesion of the polyvalent metal compound (B) to acoating surface tends to be lowered. No particular limitation is imposedon the total amount of the polyvalent metal compound, the resin, and anadditive (other than the resin) in the coating liquid, but, from theviewpoint of coating performance, the total amount is preferably 0.1 wt.% to 50 wt. %, more preferably 1 wt. % to 50 wt. %. A preferred mode ofthe layer (b) formed of the polyvalent metal compound (B) is apolyvalent-metal-compound-containing resin layer formed throughapplication of the coating liquid containing the aforementionedpolyvalent-metal-compound-containing resin.

When the coating liquid containing a poly(carboxylic acid) polymer (A)and a solvent, or the coating liquid containing a polyvalent metalcompound (B) and a solvent is applied onto the support film, an adhesivemay be applied in advance onto the surface of the support film, in orderto enhance adhesion between the support film and the layer (a) or thelayer (b). When the layer (a) or the layer (b) is provided so as to comeinto contact with an additional layer other than the support film, orwhen the multi-layer film of the present invention is designed inconsideration of contact of the multi-layer film with an additionallayer other than the support film, in order to enhance adhesion betweenthe additional layer and the layer (a) or the layer (b), a tackifier oran adhesive may be applied onto the outer surface of the layer (a) orthe layer (b). No particular limitation is imposed on the adhesive to beemployed, and specific examples of the adhesive include solvent-solubleresins employed for dry laminating, anchor coating, or primers, such asalkyd resin, melamine resin, acrylic resin, nitrocellulose, urethaneresin, polyester resin, polyether resin, phenolic resin, amino resin,fluorocarbon resin, and epoxy resin.

The multi-layer film of the present invention may be further laminatedwith an additional layer. An additional layer may be provided on thesurface of the support film on which no layer has been laminated, or anadditional layer may be provided on the surface of the layer (a) or thelayer (b) on which no layer has been laminated. Specifically, themulti-layer film may have, for example, the following layer structure:(additional layer/support film/layer (a)/layer (b)), (additionallayer/support film/additional layer/layer (a)/layer (b)), (supportfilm/layer (a)/layer (b)/additional layer), (support film/layer(b)/layer (a)/additional layer), (support film/additional layer/layer(b)/layer (a)/layer (b)/additional layer), (additional layer/supportfilm/layer (b)/layer (a)/layer (b)), or (support film/layer (b)/layer(a)/layer (b)/additional layer). The location of such an additionallayer is not limited by these examples. The material of such anadditional layer may be selected from among materials which can beemployed for forming the support film. However, the additional layermaterial is not necessarily heat-shrinkable, so long as the resultantmulti-layer film does not lose heat shrinkability. Examples of theadditional layer material include materials having printability,materials having resistance to hard use, the aforementioned adhesives,tackifiers, and heat-sensitive tackifiers. One or more additional layersmay be laminated on the multi-layer film in accordance with purposes;for example, the purposes of imparting, to the multi-layer film orsheet, strength, sealability (particularly for prevention of gasinvasion through an end portion of the film), easy-to-open property,good appearance, light shielding property, and moisture-barrierproperty. Lamination of the additional layer(s) is performed throughcoating of a layer material, or through a known lamination method by useof a film-like or sheet-like layer material with or without use of anadhesive. Specific examples of the lamination method include a drylamination method, a wet lamination method, and an extrusion laminationmethod.

The thus-produced heat-shrinkable multi-layer film of the presentinvention exhibits gas-barrier property, as well as heat shrinkability.The oxygen permeability of the multi-layer film as measured at 30° C.and a relative humidity of 80% is preferably 500 cm³/(m²·day·MPa) orless, more preferably 100 cm³/(m²·day·MPa) or less. After an object hasbeen packaged with the multi-layer film of the present invention, andthe resultant packaged product has been subjected to thermal shrinkagetreatment, the oxygen permeability of the multi-layer film as measuredat 30° C. and a relative humidity of 80% is preferably 500cm³/(m²·day·MPa) or less, more preferably 100 cm³/(m²·day·MPa) or less.As described above, thermal shrinkage treatment can be performed by useof, for example, hot water, vapor, steam, or hot air.

The multi-layer film of the present invention, which includes theheat-shrinkable support film coated with the layer (a) and the layer(b), exhibits sufficient heat shrinkability, which is attributed to theheat shrinkability of the support film (base film). The oxygenpermeability of the multi-layer film after thermal shrinkage as measuredat 30° C. and a relative humidity of 80% is preferably equal to or lowerthan that thereof before thermal shrinkage. The result indicates thateven when the film is subjected to thermal shrinkage, at leastgas-barrier property of the multi-layer film is not deteriorated.Particularly preferably, the layer (b) formed of a polyvalent metalcompound (B) is a polyvalent-metal-compound-containing resin layer.

Another embodiment of the heat-shrinkable multi-layer film according tothe present invention includes a heat-shrinkable support film (basefilm); and, on at least one surface of the base film, at least one layerstructure including a layer (a) formed of a poly(carboxylic acid)polymer (A), and a polyvalent-metal-compound-containing resin layerformed of a polyvalent metal compound (B) and a resin, the layer (a) andthe resin layer being in contact with each other, wherein themulti-layer film exhibits a percent thermal shrinkage of 90% or less,and the base film exhibits a percent thermal shrinkage of 3 to 90%. Thesame descriptions as described in relation to the above multi-layer filmcan be applied to this multi-layer film, which includes the base film,the layer (a) formed of a poly(carboxylic acid) polymer (A), and thepolyvalent-metal-compound-containing resin layer formed of a polyvalentmetal compound (B) and the resin, except that the percent thermalshrinkage of this multi-layer film is less than 3%.

The heat-shrinkable multi-layer film of the present invention can beemployed as a material for forming a bag, a label, a cover, a sheet, ora container; or can be formed into a packaging bag or a packagingcontainer. Specific examples of the form of the packaging bag include apillow packaging bag, a three-sides-sealed packaging bag, afour-sides-sealed packaging bag, and a gazette four-sides-sealedpackaging bag. Specific examples of the form of the packaging containerinclude a bottle, a tray, a cup, and a tube. For example, a containerfilled with an object to be packaged is entirely or partially coveredwith the multi-layer film of the present invention or with a packagingbag formed of the multi-layer film, and the resultant product issubjected to thermal shrinkage treatment, so as to shrink themulti-layer film or the packaging bag and to bring the film or the baginto close contact with the container, whereby the resultant containerensures oxygen gas-barrier property. The multi-layer film of the presentinvention may be laminated with another film, and the resultant productmay be employed as a cover material for a container (e.g., a tray or acup). When the materials of the multi-layer film are appropriatelychosen without departing from the scope of the present invention, themulti-layer film can be provided with, for example, easy-to-openproperty, easy-to-tear property, shrinkability, applicability tomicrowave ovens, UV-shielding property, or good appearance. Particularlywhen the multi-layer film is employed as a heat-shrinkable label, thelabel is often used for covering only the body of a bottle (e.g., a PETbottle) or a container. In this case, a tackifier (e.g., aheat-sensitive tackifier) is applied to the surface of the label thatcomes into contact with the container, and the label is caused to adhereto the container such that the label can effectively exhibit oxygengas-barrier property. When the multi-layer film is employed as, forexample, a label for a bottle (packaging container), preferably, aheat-sensitive tackifier is applied to the entirety of the label(including a peripheral portion which comes into contact with thepackaging container, and a perforated portion).

As used herein, the term “heat-sensitive tackifier” refers to a materialwhich is non-tacky at ambient temperature but exhibits tackiness underheating, and which maintains tackiness for a while after removal of aheating source. Examples of the heat-sensitive tackifier include adelayed tack agent formed of a thermoplastic resin, a solid plasticizer,and an adhesion-imparting agent; and a hot melt adhesive formed of athermoplastic resin, a wax, and an adhesion-imparting agent. From theviewpoint of prevention of, for example, exfoliation of a label, adelayed tack agent, which maintains tackiness even after cooling, ismore preferably employed. Examples of the delayed tack agent include anEVA-based agent, an acrylic agent, and a rubber-based agent.

In the present invention, a layer containing such a heat-sensitivetackifier and a polyvalent metal compound is also an embodiment of thepolyvalent-metal-compound-containing resin layer. When the multi-layerfilm including such a heat-sensitive tackifier-containing layer isemployed as a gas-barrier heat-shrinkable label for covering a container(e.g., a PET bottle), the container can be covered with the labelthrough, for example, the following procedure: a sheet-form, tube-form,or bag-form label is attached to the container, and the label-attachedcontainer is caused to pass through a shrink tunnel in which thermalshrinkage treatment is performed through, for example, steam blowing orhot air blowing, to thereby heat-shrink the label, and simultaneouslycause the label to adhere to the container on the basis of tackinessprovided by the heat-sensitive tackifier.

The multi-layer film of the present invention is suitable for use as apackaging material, packaging container, or vacuum heat-insulatingmaterial for articles which are readily impaired by, for example,oxygen, including foods, beverages, chemicals, drugs, and precisionmetallic parts such as electronic parts. In addition, the multi-layerfilm is suitable for use as a packaging material for articles whichrequire long-term reliable gas-barrier property, and require treatmentin high-temperature hot water (e.g., boiling or retort sterilization).

Examples of food-related uses of the multi-layer film include packagingmaterials for convenience-store-related products, such as packed lunch,prepared food, cooked noodle, and nabeyaki udon (pot-boiled noodle);cover material of a container for pudding or fruit jelly; packagingmaterials for Chinese foods, and general prepared foods such astsukudani (food boiled in soy sauce), pickles, and nimame (boiledbeans); packaging materials for retort foods, Japanese confectionery,Western confectionery, processed marine products, processed meatproducts, fried foods, and fish-based paste products such as kamaboko(fish sausage) and oden (Japanese hotchpotch) ingredients; packagingmaterials for meats and fish-related products; and packaging materialsfor mushroom/vegetable-related products such as raw shiitake mushroom,maitake (Grifola frondosa), apple, banana, pumpkin, ginger, and myoga(Japanese ginger). The multi-layer film is also employed for single-,bundle-, or integrated-packaging of a beverage contained in a papercontainer, such as juice, milk, or a lactic acid bacteria beverage; oremployed for single-, bundle-, or integrated-packaging of a beverage orfood contained in a plastic container, such as a PET bottle beverage,juice, milk, a lactic acid bacteria beverage, or a pot noodle product.

Other examples of uses of the multi-layer film include packagingmaterials for paper products such as data sheets, photosensitive papersheets, paper trays, and bag-in-box; packaging materials for electricappliances, electric products, mechanical components, and buildingmaterials such as plywood, floor material, ceiling material, storm door,shutter, gate, fence, and stocker; packaging materials for furniture,office machinery, fiber, metallic coils, cutting boards, tableware, andhousehold groceries such as aluminum foil; packaging materials fordoughnut-shaped products such as pipe products, electric wire products,tube products, string products, band products, and electromagnetic waveshielding tube products; packaging materials for stationery such asnotebooks, albums, and calendars; packaging materials for chemicals suchas drugs, spray products (aerosol), and detergents; packaging materialsfor cosmetic and toiletry products such as hair-related products, soap,toothpaste, and wet tissue; packaging materials for audiovisual-relatedproducts such as CDs, cassette tapes, and videotapes; packagingmaterials for earthenware; packaging materials for sporting goods,fishing goods, building materials such as columns, precision parts, andgasoline tanks; and a material for integrated packaging of drybatteries. The multi-layer film can be employed as a label for theaforementioned various packaging materials. Particularly, themulti-layer film is suitable for use as a label of, for example, a papercontainer for storing juice, milk, a lactic acid bacteria beverage, orthe like, or a PET container (bottle) for storing a beverage.

EXAMPLES

The present invention will next be described in detail by way ofExamples, which should not be construed as limiting the inventionthereto. Evaluation methods and the Examples will be described below.

1. Evaluation Methods for Heat-Shrinkable Multi-Layer Film and theExamples

1.1 Measurement of Percent Thermal Shrinkage

A film sample having dimensions of 10 cm×10 cm was immersed in hot waterat 90° C. for 30 seconds, and then the percent thermal shrinkage of thefilm sample was calculated by use of the following formula:

percent thermal shrinkage (%)={(L−L′)/L}×100 (wherein L and L′ representthe lengths of the sample in a machine direction or a transversedirection before thermal shrinkage and after thermal shrinkage,respectively). The thus-determined percent thermal shrinkage in at leastone direction of the film of the present invention is 3 to 90%.

1.2 Evaluation of Appearance

A heat-shrinkable multi-layer film was subjected to 10% shrinkage in atransverse direction through the below-described procedure under dryheating or in hot water, and the appearance of the thus-shrunk film wasevaluated on the basis of the below-described criteria.

1.2.1 10% Shrinkage Under Dry Heating

A steel can (outer diameter: 53 mm, volume capacity: 250 cm³) wascovered with a heat-shrinkable multi-layer film which had been formedinto a tubular shape such that when the film undergoes 10% shrinkage ina transverse direction, the film adheres to the steel can. Subsequently,the film-covered steel can was allowed to stand for one minute in a Geeroven whose temperature was regulated to 95° C., and was exposed to hotair, to thereby heat-shrink the film.

1.2.2 10% Shrinkage in Hot Water

A steel can (outer diameter: 53 mm, volume capacity: 250 cm³) wascovered with a heat-shrinkable multi-layer film which had been formedinto a tubular shape such that when the film undergoes 10% shrinkage ina transverse direction, the film adheres to the steel can. Subsequently,the film-covered steel can was immersed in hot water at 90° C. for 30seconds, to thereby heat-shrink the film. The appearance of theheat-shrinkable multi-layer film which had undergone 10% thermalshrinkage in a transverse direction was evaluated.

1.2.3 Evaluation Criteria

Appearance of the thus-shrunk film was evaluated on the basis of thefollowing criteria:

A: neither wrinkles nor slack is formed on the multi-layer film;cracking, breakage, and exfoliation do not occur in a coating layer; andthe coating layer maintains transparency.

B: neither wrinkles nor slack is formed on the multi-layer film, but acoating layer exhibits impaired transparency.

C: wrinkles or slack is formed on the multi-layer film; cracking,breakage, or exfoliation occurs in a coating layer; and the coatinglayer exhibits impaired transparency.

1.3 Oxygen Permeability Before and After Shrinkage

Oxygen permeability of a heat-shrinkable multi-layer film was measuredbefore and after thermal shrinkage of the film. Thermal shrinkage of thefilm was performed under dry heating and in hot water under theabove-described 10% thermal shrinkage conditions.

The oxygen permeability of the film was measured by use of an oxygenpermeability measuring apparatus (OXTRAN™ 2/20, product of ModernControl) under the following conditions: temperature: 30° C. andrelative humidity (RH): 80%. The oxygen permeability was measured inaccordance with JIS K-7126, B method (equal-pressure method) and themethod specified by ASTM D3985-81. The thus-measured value isrepresented in units of cm³ (STP)/(m²·day·MPa). As used herein, “(STP)”refers to standard conditions (0° C., 1 atm) for specifying the volumeof oxygen.

Example 1

A commercially available adhesive for anchor coating (AC) (Dicdry™LX747, product of Dainippon Ink and Chemicals, Inc., curing agent: KX75,solvent: ethyl acetate) was applied onto a heat-shrinkable polyesterfilm (Space Clean S7542, product of Toyobo Co., Ltd., thickness: 45 μm,oxygen permeability: 600 cm³ (STP)/(m²·day·MPa), percent shrinkage in amachine direction: 5%, percent shrinkage in a transverse direction: 60%(as measured after immersion in hot water at 90° C. for 30 seconds))(hereinafter the film may be referred to as a “heat-shrinkable PETfilm”) by use of a bar coater (K303PROOFER, product of RK PRINT-COATINSTRUMENT), followed by drying of the adhesive. The resultant coatinglayer was found to have a thickness of 1.0 μm. Separately, apoly(carboxylic acid) polymer (polyacrylic acid (PAA), Aron A-10H,product of Toagosei Co., Ltd., number average molecular weight: 200,000,25 wt. % aqueous solution) (hereinafter the polymer may be referred toas “PAA”) was diluted with distilled water, to thereby prepare a 5 wt. %aqueous solution. The thus-prepared aqueous solution was applied ontothe AC adhesive coating layer by use of the aforementioned bar coater,followed by drying. The resultant coating layer was found to have athickness of 0.3 μm.

Subsequently, in a manner similar to that described above, azinc-oxide-fine-particles-containing polyester resin (ZR133, product ofSumitomo Osaka Cement Co., Ltd., average particle size: 0.02 μm, solidcontent: 33 wt. %, ratio by weight of zinc oxide to resin: 1.5, solventfor dispersion (toluene:MEK=4:1)) was applied onto the above-dried PAAlayer, followed by drying. The resultant coating layer was found to havea thickness of 1.0 μm. Thus, there was produced a multi-layer filmhaving the following layer structure: heat-shrinkable PET film (45μm)/AC adhesive layer (1.0 μm)/PAA layer (0.3 μm)/ZnO-containing resinlayer (abbreviated as “ZnOA” in the Tables, wherein A refers to a resinlayer) (1.0 μm). The ratio of the thickness of the gas-barrier layer tothat of the base film was found to be 0.03. When the thus-producedmulti-layer film was immersed in hot water at 90° C. for 30 seconds, thefilm exhibited a percent shrinkage of 5% in a machine direction and apercent shrinkage of 60% in a transverse direction.

Example 2

In place of the AC adhesive, the zinc-oxide-fine-particles-containingresin (ZR133) employed in Example 1 was applied onto the aforementionedheat-shrinkable PET film in a manner similar to that of Example 1,followed by drying. The resultant coating layer was found to have athickness of 1.0 μm. Subsequently, the poly(carboxylic acid) polymer(PAA) employed in Example 1 was applied onto the coating layer, and thenthe aforementioned zinc-oxide-fine-particles-containing resin (ZR133)was applied onto the resultant PAA layer, followed by drying, to therebyproduce a multi-layer film having the following layer structure:heat-shrinkable PET film (45 μm)/ZnO-containing resin layer (1.0 μm)/PAAlayer (0.3 μm)/ZnO-containing resin layer (1.0 μm). The ratio of thethickness of the gas-barrier layer to that of the base film was found tobe 0.05. When the thus-produced multi-layer film was immersed in hotwater at 90° C. for 30 seconds, the film exhibited a percent shrinkageof 5% in a machine direction and a percent shrinkage of 60% in atransverse direction.

Example 3

The procedure of Example 1 was repeated, except that an adhesive foranchor coating was not employed, and the thickness of azinc-oxide-fine-particles-containing resin coating layer was changed.Specifically, PAA was applied onto the heat-shrinkable PET film, and thezinc-oxide-fine-particles-containing resin (ZR133) was applied onto theresultant PAA layer, followed by drying, to thereby produce amulti-layer film having the following layer structure: heat-shrinkablePET film (45 μm)/PAA layer (0.3 μm)/ZnO-containing resin layer (3.0 μm).The ratio of the thickness of the gas-barrier layer to that of the basefilm was found to be 0.07. When the thus-produced multi-layer film wasimmersed in hot water at 90° C. for 30 seconds, the film exhibited apercent shrinkage of 5% in a machine direction and a percent shrinkageof 60% in a transverse direction.

Example 4

The procedure of Example 2 was repeated, except that a heat-shrinkablepolyamide film (BONYL SC, product of Kohjin Co., Ltd., thickness: 15 μm,percent shrinkage in a machine direction: 20%, percent shrinkage in atransverse direction: 20% (as measured after immersion in hot water at90° C. for 30 seconds), oxygen permeability: 1,250 cm³(STP)/(m²·day·MPa)) (hereinafter the film may be referred to as a“heat-shrinkable ONy film”) was employed as a support film.Specifically, the zinc-oxide-fine-particles-containing resin (ZR133)employed in Example 1 was applied onto the heat-shrinkable ONy film,followed by drying. The resultant coating layer was found to have athickness of 1.0 μm. Subsequently, the PAA employed in Example 1 wasapplied onto the coating layer, and then the aforementionedzinc-oxide-fine-particles-containing resin (ZR133) was applied onto theresultant PAA layer, followed by drying, to thereby produce amulti-layer film having the following layer structure: heat-shrinkableONy film (15 μm)/ZnO-containing resin layer (1.0 μm)/PAA layer (0.3μm)/ZnO-containing resin layer (1.0 μm). The ratio of the thickness ofthe gas-barrier layer to that of the base film was found to be 0.15.When the thus-produced multi-layer film was immersed in hot water at 90°C. for 30 seconds, the film exhibited a percent shrinkage of 20% in amachine direction and a percent shrinkage of 20% in a transversedirection.

Example 5

In a manner similar to that of Example 1, the adhesive for anchorcoating (AC) and PAA were applied onto the heat-shrinkable PET filmemployed in Example 1, followed by drying. Subsequently, a mixture of anaqueous polyvinyl alcohol (PVA) solution (solid content: 10 wt. %) andan aqueous calcium acetate solution (containing calcium acetate producedby Wako Pure Chemical Industries, Ltd., concentration: 1.0 mol/kg) wasapplied onto the above-dried PAA layer in a manner similar to thatdescribed above, followed by drying. The resultant coating layer wasfound to have a thickness of 1.0 μm. Subsequently, a polyurethane (PU)resin (NEWLP SUPER RT medium, product of Toyo Ink Mfg. Co., Ltd., curingagent: VM Hardener XB, solvent: toluene, MEK) was applied onto thecalcium-acetate-containing PVA coating layer, for the purpose ofimparting water resistance. Thus, there was produced a multi-layer filmhaving the following layer structure: heat-shrinkable PET film (45μm)/AC adhesive layer (1.0 μm)/PAA layer (0.3μm)/calcium-acetate-containing PVA layer (1.0 μm)/PU resin layer (1.0μm). The ratio of the thickness of the gas-barrier layer to that of thebase film was found to be 0.03. When the thus-produced multi-layer filmwas immersed in hot water at 90° C. for 30 seconds, the film exhibited apercent shrinkage of 5% in a machine direction and a percent shrinkageof 60% in a transverse direction.

Example 6

In a manner similar to that of Example 3, an adhesive for anchor coatingwas not employed, and PAA was applied onto the heat-shrinkable PET film.Subsequently, in place of the zinc-oxide-fine-particles-containingresin, zinc oxide fine particles (ZS303, product of Sumitomo OsakaCement Co., Ltd., average particle size: 0.02 μm, solid content (zincoxide): 32 wt. %, solvent for dispersion (toluene)) were applied ontothe resultant PAA layer, followed by drying. Subsequently, apolyurethane (PU) resin (NEWLP SUPER RT medium, product of Toyo Ink Mfg.Co., Ltd., curing agent: VM Hardener XB, solvent (toluene, MEK)) wasapplied onto the zinc oxide coating layer, for the purpose of impartingwater resistance. Thus, there was produced a multi-layer film having thefollowing layer structure: heat-shrinkable PET film (45 μm)/PAA layer(0.3 μm)/zinc-oxide-fine-particles-containing layer (abbreviated as“ZnOB” in the Tables) (1.0 μm)/PU resin layer (1.0 μm). The ratio of thethickness of the gas-barrier layer to that of the base film was found tobe 0.03. When the thus-produced multi-layer film was immersed in hotwater at 90° C. for 30 seconds, the film exhibited a percent shrinkageof 5% in a machine direction and a percent shrinkage of 60% in atransverse direction.

Comparative Example 1

The zinc-oxide-fine-particles-containing resin (ZR133) employed inExample 1 was applied onto the surface of the heat-shrinkable PET filmemployed in Example 1, followed by drying, to thereby produce amulti-layer film having the following layer structure: heat-shrinkablePET film (45 μm)/ZnO-containing resin layer (1.0 μm). When thethus-produced multi-layer film was immersed in hot water at 90° C. for30 seconds, the film exhibited a percent shrinkage of 5% in a machinedirection and a percent shrinkage of 60% in a transverse direction.

Table 1 shows the results of evaluation of the multi-layer filmsproduced in Examples 1 through 6 and Comparative Example 1. TABLE 1Oxygen permeability Appearance evaluation (cm³(STP)/m² · day · MPa)After 10% After 10% shrinkage After 10% shrinkage After 10% under dryshrinkage in Before under dry shrinkage in Layer structure heating hotwater shrinkage heating hot water Ref. Ex. 1 HSPET*¹ A A 600 600 600(45) μm Ref. Ex. 2 HSONy*² A A 1250 1250 1250 (15) μm Ex. 1HSPET/AC*³/PAA*⁴/ZnOA*⁵ A A 60 40 40 (45)/(1.0)/(0.3)/(1.0) μm Ex. 2HSPET/ZnOA/PAA/ZnOA A A 40 10 10 (45)/(1.0)/(0.3)/(1.0) μm Ex. 3HSPET/PAA/ZnOA A A 60 40 40 (45)/(0.3)/(3.0) μm Ex. 4HSONy*²/ZnOA/PAA/ZnOA A A 60 40 40 (15)/(1.0)/(0.3)/(1.0) μm Ex. 5HSPET/AC/PAA/CaAc*⁶/PU*⁷ A A 60 40 40 (45)/(1.0)/(0.3)/(1.0)/(1.0) μmEx. 6 HSPET/PAA/ZnOB*⁸/PU A A 200 200 200 (45)/(0.3)/(1.0)/(1.0) μmComp. Ex. 1 HSPET/ZnOA A A 600 600 600 (45)/(1.0) μmHSPET*¹: Heat-shrinkable polyester (Space Clean S7542, product of ToyoboCo., Ltd., thickness: 45 μm)HSONy*²: Heat-shrinkable polyamide (BONYL SC, product of Kohjin Co.,Ltd., thickness: 15 μm)AC*³: Adhesive for anchor coating (Dicdry ™ LX747, product of DainipponInk and Chemicals, Inc., curing agent: KX75, solvent: ethyl acetate)PAA*⁴: Polyacrylic acid (PAA) (Aron A-10H, product of Toagosei Co.,Ltd., number average molecular weight: 200,000, 25 wt. % aqueoussolution diluted with distilled water)ZnOA*⁵: Zinc-oxide-fine-particles-containing resin (ZR133, product ofSumitomo Osaka Cement Co., Ltd., average particle size: 0.02 μm, solidcontent: 33 wt. %, solvent for dispersion: toluene, MEK)CaAc*⁶: Polyvinyl alcohol containing calcium acetate (product of WakoPure Chemical Industries, Ltd.)PU*⁷: Polyurethane resin (NEWLP SUPER RT medium, product of Toyo InkMfg. Co., Ltd., curing agent: VM Hardener XB, solvent: toluene, MEK)ZnOB*⁸: Zinc oxide fine particles (ZS303, product of Sumitomo OsakaCement Co., Ltd., average particle size: 0.02 μm, solid content: 32 wt.%, solvent for dispersion: toluene)2. Examples in Which a Heat-Shrinkable Multi-Layer Film is Employed as aLabel for a PET Bottle, and is Subjected to Evaluation

In the below-described Examples, tests were performed with the intentionof using a heat-shrinkable multi-layer film for improving gas-barrierproperty of a container (e.g., a packaging container) containing anobject. In the Examples, in order to evaluate a heat-shrinkablemulti-layer film of the present invention, a packaging container waspartially covered with the multi-layer film; specifically, themulti-layer film was employed as a label, and the body of a PET bottlewas partially covered with the film. In order to evaluate gas-barrierproperty of the heat-shrinkable multi-layer film, the film was combinedwith the packaging container by use of a heat-sensitive tackifier.

2.1 Evaluation of Appearance

A heat-shrinkable multi-layer film was combined with a PET bottle(volume capacity: 500 cm³), and the film was subjected to 10% shrinkagein a transverse direction through the below-described procedure underdry heating, followed by evaluation of the appearance of the thus-shrunkfilm.

2.1.1 10% Shrinkage Under Dry Heating

A PET bottle was covered with a heat-shrinkable multi-layer film whichhad been formed into a tubular shape such that when the film undergoes10% shrinkage in a transverse direction, the film adheres to the PETbottle. Subsequently, the film-covered PET bottle was allowed to standfor one minute in a Geer oven whose temperature was regulated to 95° C.,and was exposed to hot air, to thereby heat-shrink the film. Thermalshrinkage was performed such that the surface area of a region of thePET bottle covered with the thus-shrunk multi-layer film accounts forabout 80% of the entire surface area of the PET bottle. The appearanceof the heat-shrinkable multi-layer film which had undergone 10% thermalshrinkage in a transverse direction and which had been combined with thePET bottle was evaluated.

2.1.2 Evaluation Criteria

A: neither wrinkles nor slack is formed on the multi-layer film; thefilm adheres to the container; cracking, breakage, and exfoliation donot occur in a coating layer; and the coating layer maintainstransparency.

B: neither wrinkles nor slack is formed on the multi-layer film, and thefilm adheres to the container, but a coating layer exhibits impairedtransparency.

C: wrinkles or slack is formed on the multi-layer film; the film doesnot adhere to the container; cracking, breakage, or exfoliation occursin a coating layer; and the coating layer exhibits impairedtransparency.

2.2 Oxygen Permeability After Shrinkage

The oxygen permeability of the PET bottle which had been combined withthe multi-layer film by use of a tackifier was measured through thefollowing two methods: a method in which the bottle per se is subjectedto measurement (hereinafter this method will be referred to as “thepackage method”); and a method in which a portion of the bottle that iscovered with the multi-layer film is cut out of the bottle, and the cutportion is subjected to measurement (hereinafter this method will bereferred to as “the film method”). In the package method, the bottle wasfixated in a chamber (temperature: 30° C., relative humidity: 30%),nitrogen (relative humidity: 80%) was caused to flow through the bottle,and the amount of oxygen which permeates from the outside (airatmosphere) of the bottle into the bottle was measured by use of theoxygen permeability measuring apparatus employed in 1.3 described above.The thus-measured value was multiplied by five, and the resultant valuewas regarded as the value in the case where a space outside the bottle(inside the chamber) is filled with 100% oxygen; i.e., the oxygenpermeability of the bottle. In the film method, the oxygen permeabilitywas measured in a manner similar to that described above in 1.3. Theoxygen permeability measured through the package method is representedin units of cm³(STP)/(bottle·day·MPa), whereas the oxygen permeabilitymeasured through the film method is represented in units of cm³(STP)/(m²day·MPa)

Example 7

A heat-sensitive tackifier (Heat Magic DW4070, product of Toyo Ink Mfg.Co., Ltd.) was applied onto the surface of the ZnO-containing resinlayer (opposite the surface of the heat-shrinkable PET film) of themulti-layer film of Example 2, such that the multi-layer film adheres toan object to be packaged during the course of thermal shrinkage of thefilm, followed by drying of the adhesive. The resultant heat-shrinkablemulti-layer film (percent shrinkage in a machine direction: 5%, percentshrinkage in a transverse direction: 60% (as measured after immersion inhot water at 90° C. for 30 seconds)) was formed into a cylindrical shapesuch that the adhesive layer faces inward. The body of a commerciallyavailable 500-mL PET bottle was covered with the thus-formed cylindricalmulti-layer film. Subsequently, the film-covered PET bottle was allowedto stand for one minute in a Geer oven whose temperature was regulatedto 95° C., and was exposed to hot air, to thereby heat-shrink the filmand combine the film with the bottle.

Thus, there was produced a packaging container having the followingstructure: heat-shrinkable PET film (45 μm)/ZnO-containing resin layer(1.0 μm)/PAA layer (0.3 μm)/ZnO-containing resin layer (1.0μm)/tackifier layer (3.0 μm)/PET bottle (average thickness: 350 μm). Thesurface area of the multi-layer film combined with the PET bottlesurface was found to account for 80% of the entire surface area of thePET bottle. The oxygen permeability of the PET bottle per se was foundto be 2.0 cm³ (STP)/(bottle·day·MPa) at 30° C. and a relative humidityof 80%.

Example 8

The heat-sensitive tackifier employed in Example 7 was applied onto thesurface of the heat-shrinkable PET film of the multi-layer film ofExample 2, such that the multi-layer film adheres to an object to bepackaged during the course of thermal shrinkage of the film, followed bydrying of the adhesive. The resultant heat-shrinkable multi-layer film(percent shrinkage in a machine direction: 5%, percent shrinkage in atransverse direction: 60% (as measured after immersion in hot water at90° C. for 30 seconds)) was formed into a cylindrical shape such thatthe tackifier layer faces inward. In a manner similar to that of Example7, the body of a commercially available 500-mL PET bottle was coveredwith the thus-formed cylindrical multi-layer film. Subsequently, thefilm-covered PET bottle was allowed to stand for one minute in a Geeroven whose temperature was regulated to 95° C., and was exposed to hotair, to thereby heat-shrink the film and combine the film with thebottle. Thus, there was produced a packaging container having thefollowing structure: ZnO-containing resin layer (1.0 μm)/PAA layer (0.3μm)/ZnO-containing resin layer (1.0 μm)/heat-shrinkable PET film (45μm)/tackifier layer (3.0 μm)/PET bottle (average thickness: 350 μm) Thesurface area of the multi-layer film combined with the PET bottlesurface was found to account for 80% of the entire surface area of thePET bottle.

Comparative Example 2

The heat-sensitive tackifier employed in Example 7 was applied onto theheat-shrinkable PET film employed in Example 1, followed by drying. Theresultant heat-shrinkable multi-layer film (percent shrinkage in amachine direction: 5%, percent shrinkage in a transverse direction: 60%(as measured after immersion in hot water at 90° C. for 30 seconds)) wasformed into a cylindrical shape such that the adhesive layer facesinward. In a manner similar to that of Example 7, the body of acommercially available 500-mL PET bottle was covered with thethus-formed cylindrical multi-layer film. Subsequently, the film-coveredPET bottle was allowed to stand for one minute in a Geer oven whosetemperature was regulated to 95° C., and was exposed to hot air, tothereby heat-shrink the film and combine the film with the bottle. Thus,there was produced a packaging container having the following structure:heat-shrinkable PET film (45 μm)/tackifier layer (3.0 μm)/PET bottle(average thickness: 350 μm). The surface area of the heat-shrinkable PETfilm combined with the PET bottle surface was found to account for 80%of the entire surface area of the PET bottle.

Table 2 shows the results of evaluation of the multi-layer filmsproduced in Examples 7 and 8 and Comparative Example 2. TABLE 2 Oxygenpermeability Package method Film method Appearance (cm³(STP)/ (cm³(STP)/Layer structure evaluation bottle · day · MPa) m² · day · MPa) Ref. Ex.3 PETb*¹ — 2.0 50 (average: 350 μm) Ex. 7HSPET*²/ZnOA*³/PAA*⁴/ZnOA/Ad*⁵/PETb A 0.4 5.0(45)/(1.0)/(0.3)/(1.0)/(3.0)/(350) Ex. 8 ZnO/PAA/ZnOA/HSPET/Ad/PETb A0.2 3.0 (1.0)/(0.3)/(1.0)/(45)/(3.0)/(350) Comp. Ex. 2 HSPET/Ad/PETb A2.0 50 (45)/(3.0)/(350)PETb*¹: Commercially available PET bottle (volume capacity: 500 cm³)HSPET*²: Heat-shrinkable polyester (Space Clean S7542, product of ToyoboCo., Ltd., thickness: 45 μm)ZnOA*³: Zinc-oxide-fine-particles-containing resin (ZR133, product ofSumitomo Osaka Cement Co., Ltd., average particle size: 0.02 μm, solidcontent: 33 wt. %, solvent for dispersion: toluene, MEK)PAA*⁴: Polyacrylic acid (PAA) (Aron A-10H, product of Toagosei Co.,Ltd., number average molecular weight: 200,000, 25 wt. % aqueoussolution diluted with water)Ad*⁵: Heat-sensitive tackifier (Heat Magic DW4070, product of Toyo InkMfg. Co., Ltd.)3. Examples in Which a Heat-Shrinkable Multi-Layer Film is Employed forEntirely Covering an Object, and is Subjected to Evaluation

In the below-described Examples, in order to evaluate a heat-shrinkablemulti-layer film, an object is entirely covered with the film.Specifically, a sausage product was packaged with the heat-shrinkablemulti-layer film, and then the film was subjected to evaluation.

Example 9

Through dry lamination, a heat-shrinkable polyethylene film (Polyset UM,Product of Kohjin Co., Ltd., thickness: 35 μm, percent shrinkage in amachine direction: 15%, percent shrinkage in a transverse direction: 18%(as measured after immersion in hot water at 90° C. for 30 seconds)) wasattached, via a polyurethane adhesive (Takelac A620, product of MitsuiTakeda Chemicals, Inc., curing agent: Takenate A65, solvent: ethylacetate), onto the surface of the ZnO-containing resin layer of theheat-shrinkable multi-layer film produced in Example 4, to therebyproduce a multi-layer film having the following layer structure:heat-shrinkable ONy film (15 μm)/ZnO-containing resin layer (1.0 μm)/PAAlayer (0.3 μm)/ZnO-containing resin layer (1.0 μm)/adhesive layer (2μm)/heat-shrinkable polyethylene film (35 μm). A sausage product waspackaged with the thus-produced multi-layer film by use of a bag-formingand packaging machine (ONPACK-6600AII, product of Orihiro Co., Ltd.).The resultant packaged product was immersed in hot water at 90° C. for10 minutes, to thereby perform thermal shrinkage and simultaneouslythermal sterilization. After completion of thermal shrinkage,substantially no change in the shape of the sausage product wasobserved, and the packaged product exhibited good appearance, with thefilm being sufficiently tightly fitted to the sausage product. Nobreakage was observed in the thus-shrunk film. The oxygen permeabilityof the thus-shrunk film was found to be 1.0 cm³(STP)/(m² day·MPa) at 30°C. and a relative humidity of 80%.

Comparative Example 3

By means of dry lamination, the heat-shrinkable polyethylene filmemployed in Example 9 was attached, via the adhesive employed in Example9, onto one surface of the heat-shrinkable ONy film employed in Example4, to thereby produce a multi-layer film having the following layerstructure: heat-shrinkable ONy film (15 μm)/adhesive layer (2μm)/heat-shrinkable polyethylene film (35 μm). A sausage product waspackaged with the thus-produced multi-layer film by use of thebag-forming and packaging machine described above in Example 9. Thesausage-packaged product was immersed in hot water at 90° C. for 10minutes, to thereby perform thermal shrinkage and simultaneously thermalsterilization. After completion of thermal shrinkage, substantially nochange in the shape of the sausage product was observed, and thepackaged product exhibited good appearance, with the film beingsufficiently tightly fitted to the sausage product. No breakage wasobserved in the thus-shrunk film. The oxygen permeability of thethus-shrunk film was found to be 1,200 cm³(STP)/(m²·day·MPa) at 30° C.and a relative humidity of 80%.

INDUSTRIAL APPLICABILITY

The present invention provides a heat-shrinkable multi-layer film whichis a packaging material exhibiting gas-barrier property, and which isemployed for shrink packaging of an object without use of a container ora cover material, or for shrink packaging of the entirety of acontainer.

1. A heat-shrinkable multi-layer film comprising a heat-shrinkable basefilm and, on at least one surface of the base film, at least one layerstructure including a layer (a) formed of a poly(carboxylic acid)polymer (A) and a layer (b) formed of a polyvalent metal compound (B),the layers (a) and (b) being in contact with each other, and themulti-layer film exhibiting a percent thermal shrinkage of 3 to 90%. 2.A heat-shrinkable multi-layer film according to claim 1, wherein theheat-shrinkable base film exhibits a percent thermal shrinkage of 3 to90%.
 3. A heat-shrinkable multi-layer film according to claim 1, whichexhibits a percent thermal shrinkage of 5 to 90%.
 4. A heat-shrinkablemulti-layer film according to claim 1, wherein the layer (b) formed ofthe polyvalent metal compound (B) is apolyvalent-metal-compound-containing resin layer formed of thepolyvalent metal compound (B) and a resin.
 5. A heat-shrinkablemulti-layer film according to claim 1, wherein the ratio of the totalthickness of a gas-barrier layer formed of the layers (a) and (b) whichare in contact with each other to that of the base film is 0.001 to 0.5.6. A heat-shrinkable multi-layer film according to claim 1, whichexhibits an oxygen permeability of 500 cm³/(m²·day·MPa) or less asmeasured at 30° C. and a relative humidity cf 80%.
 7. A heat-shrinkablemulti-layer film according to claim 1, wherein the polyvalent metalcompound (B) is a divalent metal compound.
 8. A heat-shrinkablemulti-layer film according to claim 1, wherein the poly(carboxylic acid)polymer (A) is a homopolymer or copolymer formed of at least onepolymerizable monomer selected from among acrylic acid, maleic acid, andmethacrylic acid, and/or a mixture of such homopolymers or copolymers.9. A heat-shrinkable multi-layer film according to claim 1, whichcontains an additional layer.
 10. A heat-shrinkable multi-layer filmaccording to claim 9, wherein the additional layer is anadhesive-containing layer.
 11. A heat-shrinkable multi-layer filmaccording to claim 1, which, after thermal shrinkage, exhibits an oxygenpermeability equal to or lower than that before thermal shrinkage.
 12. Aheat-shrinkable multi-layer film comprising a heat-shrinkable base filmand, on at least one surface of the base film, at least one layerstructure including a layer (a) formed of a poly(carboxylic acid)polymer (A), and a polyvalent-metal-compound-containing resin layerformed of a polyvalent metal compound (B) and a resin, the layer (a) andthe resin layer being in contact with each other, wherein themulti-layer film exhibits a percent thermal shrinkage of 90% or less,and the base film exhibits a percent thermal shrinkage of 3 to 90%. 13.A packaging material comprising a heat-shrinkable multi-layer film asrecited in claim
 1. 14. A packaging material according to claim 13,which is in the form of a bag, a sheet, a label, a container, or a covermaterial.
 15. A packaged product obtained by packaging an object with aheat-shrinkable multi-layer film as recited in claim
 1. 16. A packagedproduct according to claim 15, wherein, when the product is subjected tothermal shrinkage treatment, the heat-shrinkable multi-layer filmexhibits an oxygen permeability of 500 cm³/(m²·day·MPa) or less asmeasured at 30° and a relative humidity of 80%.
 17. A heat-shrinkablelabel comprising a heat-shrinkable multi-layer film as recited inclaim
 1. 18. A heat-shrinkable label according to claim 17, to which aheat-sensitive tackifier has been applied.
 19. A packaging materialcomprising a heat-shrinkable multi-layer film as recited in claim 12.20. A packaged product obtained by packaging an object with aheat-shrinkable multi-layer film as recited in claim 12.